1. Field of the Invention
The present invention relates to a roll-paper feeding device for feeding a roll-type sheet medium such as a roll paper and an image forming apparatus provided in which the roll-paper feeding device is incorporated.
2. Description of the Related Art
Conventionally, as a method of holding a roll paper, which is a long-size paper wound around a paper tube as a core, in a paper feeding device, a spool system or a flange system is adopted to deal with various sizes of roll papers.
The spool system has a disadvantage in that a long spool has to be passed through the paper tube of the roll paper, which deteriorates user operability. On the other hand, by providing a drive transmission mechanism in the spool when a rewinding mechanism or the like is provided, the roll paper itself can be easily driven. Moreover, uplift of the roll paper during conveyance thereof can be easily pressed by engagement between a spool shaft and the drive transmission mechanism.
The flange system has very high user operability because the roll paper is held by flange members being inserted into the paper tube from both sides thereof. However, to directly transmit the drive force to the roll paper, only two approaches are available. In one approach, the diameter of the flange member is made larger than a maximum outer diameter of the roll paper to transmit the drive force to the outer diameter of the flange member. In the other approach, a baseline of a paper in its width direction upon its conveyance is set as the edge portion of the paper, so that the drive force is transmitted to the flange member.
If the outer diameter of the flange member is made larger, the diameter of the flange member is also needs to be made larger, which results in an increase of the overall size of the device. Therefore, this approach is disadvantages in that the roll paper has to be set in the device while one side of the roll paper is being held up. Moreover, if the baseline in the width direction upon the conveyance is set as the edge portion of the paper and when the long-size paper is conveyed, a problem may arise in the quality during the conveyance such as shrink, twist, or skew.
Japanese Patent Application Laid-open No. 2006-248683 describes that by holding a roll paper by a spool and using a one-way gear, the drive is transmitted to the spool only upon rewinding, so that high-accuracy conveyance is achieved.
Japanese Patent Application Laid-open No. 2003-276264 adopts a method in which the system is a flange type and the diameter of a flange member is made smaller than a maximum outer diameter of a roll paper and the roll paper is rewound by a receiving roller of the flange member. In this method, if a rewinding speed is constant, the remaining amount of the roll paper is reduced to cause a slip amount of the receiving roller to increase. Moreover, because the rewinding cannot be continuously performed, the roll paper cannot be rewound for a long distance.
To take care of the disadvantages of a sheet conveying device described in Japanese Patent Application Laid-open No. 2003-276264 and of continuously rewinding the roll paper, the applicants of the present application have proposed that the flange member is reversely rotated by a rewinding roller through the receiving roller of the flange member to allow continuous rewinding of the paper, and that drive transmission between the rewinding roller and the receiving roller and between the receiving roller and the flange member is carried out based on friction or on magnetic force and slip. By doing so, paper tension force when the roll paper is to be rewound through a conventional gear or the like allows stable rewinding without changing a rewinding speed according to a change in a roll diameter.
However, when the drive transmission is carried out by friction, because the flange member of the roll paper is just put on the receiving roller, when the roll paper is intermittently conveyed in a forward feeding direction, the roll paper is uplifted from the receiving roller being a rotation support member with a quicker rising time at which the roll paper is started to rotate, which causes unstable rotation, and brake force becomes insufficient to cause slack of the paper, which may cause the skew. Particularly, the diameter size of the paper tube of the roll paper includes 2 inches and 3 inches as general specifications. Thus, a difference occurs between their weights and between their centers of gravity, and conditions for the roll papers that do not uplift are thereby largely different from each other.
Furthermore, when the drive transmission is carried out by magnetic force, uplift of the roll paper can be prevented. However, because it is necessary to newly provide an electromagnet device, this case has disadvantages in that the device is complicated to result in a cost increase.